This invention relates to the field of polishing slurries.
In an attempt to maximize flatness for magnetic storage media such as computer hard disks, some manufacturers are considering glass substrates. Glass substrates"" polishing processes first remove the damaged-micro-scratched layer produced in lapping. This damaged layer, caused by the aftermath of lapping, has a nominal depth of a few times the final abrasive""s diameter used in its second or final stage lap or at least about 0.001xe2x80x3 (0.025 mm) per side. For example, lapping with 7 xcexcm particles requires a removal of about 20 xcexcm per side for a total of about 40 xcexcm. The polishing process must remove these damaged layers.
Cerium(IV) oxide slurries containing de-ionized water provide the bulk of the typical polish formulations for removing micro-scratched surfaces from computer disk glass substrates. For example, Chen et al., in U.S. Pat. No. 5,087,481, disclose using a xe2x80x9ccoarsexe2x80x9d ceria slurry for polishing a glass disk. These ceria formulations also involve the chemical mechanism of SiO2 catalysis to Si(OH)4 or hydrolysis. The hydrolysis of the SiO2 or glass dissolves the surface layer to remove the damaged surface. Furthermore, Lee M. Cook, in xe2x80x9cChemical Processes in Glass Polishingxe2x80x9d discloses accelerated removal rate at pH 14 when polishing with silica particles. Typical removal rates for crystalline computer glass disks with ceria-based slurries range from 0.6 to 1 micron per minute at 1-2 pounds per square inch pad pressure (6.9 to 13.8 KPa). This removal rate typically varies with polishing temperature, polishing pad pressure, and polishing pad velocity.
Mechanical enhancements that increase the removal rate include: 1) increasing the polishing pressure; 2) using larger abrasive particles; and 3) polishing at higher pad velocities. Unfortunately, increasing the removal rate with these mechanical means has a negative impact on surface finish. Furthermore, for a high throughput process, the removal rate of the damaged surface must increase without any negative impact on surface finish.
Melard et al., in U.S. Pat. No. 4,601,755, disclose a slurry adapted for glass. The slurry uses ceria and a rare earth pyrosilicate having the formula Ln2xe2x88x92xCexSi2O7, wherein Ln is at least one lanthanide or yttrium and x is a number ranging from zero to less than 2.
Commercial operations require a two-step polish for achieving a surface roughness of Ra less than 5 Angstroms (AFM). The first-step polish removes the micro-scratched layer caused by lapping. The second-step polish removes surface scratches to smooth the glass surface. Manufacturers require slurries with consistent polishing performance to optimize performance of the first-step and second-step polish.
The polishing slurry includes by weight percent, 0.1 to 50 metal oxy-acid accelerator, 0.5 to 50 cerium oxide abrasive particles, 0.1 to 2 inorganic suspension agent and balance water. The metal oxy-acid accelerator is formed with a metal selected from the group consisting of chromium, lanthanum and rare earth metals 59 to 71, manganese, molybdenum, niobium, osmium, rhenium, ruthenium, titanium, tungsten, vanadium, yttrium and zirconium.
The method of polishing glass substrates includes the steps of contacting the glass substrate with polishing slurry having the above composition. Then polishing the glass substrate with a pad; and the slurry removes the glass surface from the glass substrate.